Vaccines are being developed for S. sonnei. One way to evaluate the efficacy of such vaccines is to carry out a volunteer challenge study in which volunteers who receive the vaccine along with a group of unimmunized volunteers are given an oral challenge with live virulent bacteria. Before such studies are carried out at Johns Hopkins, the volunteer challenge model must be established and validated. Past volunteer studies at the University of Maryland have used freshly prepared bacteria for the challenge, but a challenge prepared from frozen bacteria would seem to have several advantages for vaccine evaluation because of its reproducibility. This, we plan to conduct a dose finding study using a frozen inoculumto validate the frozen challenge model of shigellosis.